The steel plates and tubes which typically provide the internally available surfaces of drumless boilers are often constructed of various steel alloys which lack copper. Alloys known to the present inventor to be frequently encountered include A515Gr70 Boiler Plate, ASTM A182F22 (A213T22)--21/4 percent Cr, ASTM A182F11 (A213T11)--11/4 percent Cr, ASTM A213T2--1/2 percent Cr, and ASTM A182F1--1/2 percent Mo.
Drumless boilers, e.g., Babcock & Wilcox Universal Pressure and Combustion Engineering supercritical units, do not circulate water in the tubes, but operate with "once-through" cycles. This fact, as well as a lack of copper-based metallurgy in the feedwater train of such boilers, and consistently high-quality water chemistry used in the operation of such boilers, causes the deposits which inevitably form in the tubes of those drumless boilers to be magnetite (Fe.sub.3 O.sub.4) of a fairly consistent composition, without the copper that is often found in the deposits that form in drum boilers.
When magnetite is dissolved in the presence of an iron surface or iron is corroded by acid, Fe(II) ions are released into solution: EQU Fe.sub.3 O.sub.4 +8H.sup.+ =2Fe.sup.+3 +Fe.sup.+2 +4H.sub.2 O (1) EQU Fe+2Fe.sup.+3 =3Fe.sup.+2 (2) EQU Fe+Fe.sub.3 O.sub.4 +8H.sup.+ =4Fe.sup.+2 +4H.sub.2 O (3) EQU Fe+2H.sup.+ =H.sub.2 +Fe.sup.+2 (4)
It is known that EDTA solvent-based cleaning solutions, e.g., solutions of (NH.sub.4).sub.4 EDTA and (NH.sub.4).sub.2 EDTA, will readily remove magnetite deposits from the internal surfaces of drumless boilers. The expense of EDTA solvents, however, has caused chemical cleaning service providers to focus on less expensive cleaning alternatives.
The Reich patent (U.S. Pat. No. 3,003,898, issued Oct. 10, 1961) discloses a method and composition for removing scale and tenacious foreign matter from the internal surfaces of metal-walled (typically steel-walled) vessels used for storing, transferring or circulating fluids. Typical are the surfaces of boiler and heat exchanger tubes, transfer lines and storage tanks. It is believed that the methods and compositions disclosed in the Reich patent were used commercially in the United States from the 1960s until 1985.
The invention claimed in Reich was predicated upon the discovery that a synergistic effect on the cleaning of scale and other adhesive foreign matter from steel surfaces apparently was obtained by using a cleaning solution comprising an aqueous solution containing between 0.2 and 20.0 percent-by-weight of a mixture of formic acid and citric acid, in which the ratio of formic acid to citric acid was between 1:6 and 3:1. Reich reported that the use of pure acids or mixtures outside the foregoing range was unacceptable because of the formation of a sludgy precipitate believed to be ferric citrate at lower ratios and hydrated ferric oxide at higher ratios. See FIG. 4 of the Reich patent which teaches that, under the conditions investigated by Reich, iron titrate precipitated from the solution if the weight ratio of formic acid to citric acid was less than 1:6, and hydrated ferric oxide precipitated from the solution if the weight ratio of formic acid to citric acid was greater than 3:1.
The apparatus used by Reich for the tests to determine the effects of aqueous cleaning solutions including formic acid, citric acid, and mixtures of the two acids was not an actual steam boiler or equivalent industrial apparatus. Reich employed a reflux condenser, apparently used without precautions to exclude air or to provide an inert or reducing atmosphere. The present inventor concludes from his reading of Reich that air was able to enter Reich's experiment; otherwise, he would not have been stabilizing ferric oxide, in which the iron is in the ferric oxidation state. Introduction of air into utility boilers is uncharacteristic of at least present day chemical solution-based cleaning of iron oxide from the internal surfaces of utility boilers and similar industrial equipment.
Reich further taught that the temperature of the aqueous acidic solutions contacting the scale should be maintained between 150.degree. F. and their boiling points, preferrably between 200.degree. F. and their boiling points. Thereafter, the solutions should be heated to at least 212.degree. F., preferably above their boiling points to decompose any remaining acid. Reich also taught that the solutions should contain between 0.1 and 1.0 percent-by-weight of a corrosion inhibitor such as those described in U.S. Pat. Nos. 2,403,153; 2,606,873; 2,510,063; and 2,758,970, all of which are incorporated herein by reference. Reich also suggested that the solutions should contain 0.01 to 0.1 percent-by-weight of a wetting agent exemplified by a condensation product produced by condensing ethylene oxide with di-secondary butylphenol in a proportion of about 10 moles of ethylene oxide to 1 mole of di-secondary butylphenol.
For ensuring adequacy of disclosure without unnecessarily lengthening this text, the specification of the Reich patent is incorporated herein by reference.
For reasons unknown to the present inventor, the scale removing chemical of choice over the last several years, at least since 1985, has been a solvent based on a mixture of glycolic acid and formic acid present in a 2:1 weight ratio and typically totaling 3.0 percent-by-weight of an aqueous solution. These glycolic acid-formic acid solutions generally also include an inhibitor and a scale removal accelerating agent.
Use of these aqueous solutions of glycolic acid-formic acid mixtures is more expensive than use of the aqueous formic acid-citric acid solutions within the concentration and proportion ranges and under the conditions taught in the Reich patent. However, both are less expensive than using EDTA-based solvents. Cleaning times using the method taught in the Reich patent tend to be comparable to those experienced using aqueous solutions of glycolic acid-formic acid mixtures as the solvent, e.g., from about 20 percent longer to about 20 percent shorter.
A strong motivation of the present inventor to re-explore the cleaning of drumless boilers using an aqueous solvent solution based on a mixture of formic acid and citric acid was the prospect of savings in chemical costs. Because formic acid is less expensive than citric and other carboxylic acids, higher ratios of formic acid to carboxylic acid offer the possibility of significant cost savings.
Among the important criteria that a chemical cleaning service provider or customer typically may specify in connection with a contract for chemically cleaning the interior of a drumless boiler are the following:
that the boiler tubes be cleaned within 30 hours or less of contact with the cleaning solution;
that the cleaning be performed at a temperature within the range between 150.degree. F. and 200.degree. F.;
that the solvent be adequately inhibited to prevent excessive attack on the bared metal of the boiler, e.g., a corrosion rate below 0.015 lb/ft.sup.2 /day (Basically the higher the temperature, the more the chromium in the alloy, the greater the acid concentration, or the higher the flow rate, the higher will be the necessary concentration of expensive corrosion inhibitors, all other factors being equal.);
that the solution be able to retain at least 0.7 percent-by-weight of iron in the ferrous state for at least 24 hours; and
that the concentrations of metals dissolved into the solution be reducible to below 1 ppm by conventional waste treatment methods, e.g., the addition of lime, caustic, peroxide or air.
The chemical cleaning industry has long sought inexpensive and effective cleaning solutions and methods meeting all of the foregoing criteria. Those needs have now been filled by the present invention.